Cause people get the same stones (exactly same kind of "stones") after taking other kinds of magnesium (Magnesium peroxide, magnesium oxide), without taking any oil.
If it was oil ... you would never get the same kind of stones from taking simply Magnesium peroxide.
On the other side, bile is excreted in both cases ... and bile can solidify inside intestines (possibly in contact with HCl from stomach), bile can appear like a gallstone ( Gallstones are composed of bile) .... and those "stones" can convert back into liquid later, as you demonstrated with your video.